Conventionally, composite materials have essentially consisted of either thermosetting or thermoplastic materials having fillers or reinforcing materials dispersed therein. The supply and cheapness of wood fiber on a world-wide basis, as well as its other advantages, has made wood fibers a desirable material for filler/reinforcing in combination with such thermosetting or thermoplastic materials.
Conventional wood composites comprised of wood fibers and thermosetting polymers, such as plywood and fiber board, which are offered on the market today tend to suffer from low moisture resistance due to the hydrophilic nature of the wood. For this reason, amongst others, during the last several years, interest has grown in the combination of wood fibers with thermoplastic matrices for the production of composite materials, particularly for low cost/high volume applications. Using a thermoplastic polymer in the composite has been found to result in an improvement in the water resistance and other properties of the composite material.
Further, the development of wood fiber and thermoplastic composites has occurred because wood-derived fillers or reinforcement for thermoplastic polymers may have several further advantages. For instance, the use of wood fibers as filler and reinforcement for thermoplastics may result in a composite material having a lower cost to produce, a lower density, a greater Romulus (which results in lower filler damage during processing), and less abrasiveness to processing equipment as compared to other fillers or reinforcing materials on the market for thermoplastics. In addition, wood fiber fillers are derived from a renewable resource.
However, the use of wood fibers in combination with thermoplastics has been limited mainly as a result of difficulties experienced in dispersing the hydrophilic wood fibers in the hydrophobic thermoplastic polymer matrix. In addition, as a result of the hydrophilic nature of the wood fibers and the hydrophobic nature of the thermoplastic, there appears to be a weak interaction therebetween. Insufficient dispersion and weak interactions may result in poor adhesion (wettability and particle-bonding), and in consequence, inferior mechanical properties in the resultant composite materials.
More particularly, because the wood filler is insoluble in the thermoplastic polymer matrix, a polymer matrix interface is created. The interface/interphase region strongly affects the end properties of the composite material. The interface is the surface between the two phases, whilst the interphase is defined as the region between the wood and the thermoplastic polymer matrix. Poor interaction between the wood and the polymer matrix decreases the interfaces between the phases.
Several approaches have been developed for improving the compatibility of the wood fibers and the thermoplastic polymer matrix. More particularly, emphasis has been placed on improving the interfacial adhesion between the hydrophilic wood particles and the hydrophobic thermoplastic polymer matrix. Two primary approaches have been used: (1) modification of the wood surface before processing; and (2) use of a compatibility or coupling agent during compounding.
Many different approaches are reported in the literature for surface treatment of the wood, for example, coating and grafting of the wood fibers. The basic method employed is to chemically modify the wood filler component prior to compounding.
With respect to the use of compatibility agents, referred to in the industry as coupling agents, it has been determined that the adhesion between the wood fibers and the synthetic polymer can be improved when the coupling agent is added during processing. Coupling agents may act to modify the interface between the wood fibers and the thermoplastic polymer by interacting therewith and forming a link or bond between the components. Thus, the use of coupling agents may enhance the inherently poor attraction between the polar wood and the non-polar thermoplastic component.
The "coupling agents" previously used in the industry cover a broad range of complex polymers, such as: maleic anhydride polyethylene (MAPE)/polypropylene (MAPP); styrene-butadiene-styrene; carboxylic wax; ionomer modified polyethylene; and low molecular weight polypropylene. However, these types of coupling agents may be expensive to use or difficult to disperse within a matrix mixture.
There is therefore a need in the industry for a composite material comprised of relatively low cost components and exhibiting comparable or improved properties as compared to known composites in the market. More particularly, there is a need for an improved composite material comprised of an organic filler and a thermoplastic polymer. There is also a need for a relatively cost effective coupling agent for use in the improved composite material to enhance the properties thereof. There is also a need for a process for producing the composite material.